1. Field of the Invention
The present invention relates to a toner suitable for electrophotography, electrostatic recording, electrostatic printing and the like, to an efficient production process for the same, and to an image forming method using the toner.
2. Description of the Related Art
Image forming based on electrophotography generally involves a series of the following individual steps: a latent electrostatic image formation step in which using a variety of means a latent electrostatic image is formed on a photosensitive layer having photoconductive substances; a developing step in which the formed latent electrostatic image is developed by application with toner to form a toner image; a transferring step in which the toner image is transferred to a recording member such as paper; a fixing step in which the toner image transferred to the recording member is fixed thereto by applying heat, pressure, heat/pressure, or solvent's vapor; and a cleaning step in which toner particles remained with the photosensitive layer are removed, for example.
It is required that toners for electrophotography be produced by more energy-saving, environment-friendly processes. The conventional method of kneading and pulverization is employed in current toner production processes.
In such toner production processes that use the kneading and pulverization method, how uniformly each constituent material is dispersed and pulverized is important to ensure that the resultant toner particles have uniform shapes. In general, toner particles have amorphous shapes with randomly-sized cross sections, and control of the shape or structure of toner particles become very difficult. Moreover, when coloring materials, releasing agents, charge-controlling agents and the like are added to the toner, these additives tend to migrate to the surfaces of toner particles during a pulverization process because they cleavage along their crystal plane, resulting in a problem that toner characteristics (e.g., flowability and charging properties) may be reduced. e.g., variations may occur in the charging properties among individual toner particles.
Thus, in recent years, liquid media-based chemical methods (polymerization methods such as emulsion polymerization method, suspension polymerization method, dispersion polymerization method, dissolution and suspension method, and dissolution, suspension and extension method) have been used in most cases for the production of toners.
In the suspension polymerization method, toner materials containing monomers, a polymerization initiator and the like are dispersed in an aqueous medium to form oil droplets, followed by heat treatment to cause a polymerization reaction to take place for the production of toner.
In the emulsion polymerization method, toner materials containing polymers and the like are mixed with an aqueous medium to form oil droplets by allowing the toner materials to be dispersed or emulsified in or with the aqueous medium for the production of toner (see Japanese Patent Application Laid-Open (JP-A) No. 05-66600 and 08-211655).
The dissolution and suspension method is disclosed in Japanese Patent (JP-B) No. 3141783, for example.
In view of recent environmental problems, as chemical toners produced through these chemical methods (polymerization methods), chemical toners termed as “capsule toner”, “core shell toner”, etc. are available that are provided in a form that makes efficient provision of desired functions possible.
Toner production processes that involve any of the foregoing polymerization methods can produce spherical toner particles that have smaller diameters and a narrower particle size distribution than those produced by toner production processes involving the pulverization method; however, it is difficult to form droplets of desired shape in the dispersion medium, the range of choice of available materials is small, and variations occur in the charging properties among individual toner particles due to variations in the toner constituting materials. In addition, delicate controlling of the degree of emulsification is required for each color toner, resulting in poor robustness in toner production.
The most challenging problem is that toner surface becomes hydrophilic because toner is produced either in water or hydrophilic medium, reducing charging properties of toner particles and environmental characteristics with time. This may cause such problems as abnormal development and transfer operations, toner splashes, or poor image quality.
Moreover, the polymerized toners described above have a basic problem that the production process thereof entails generation of a large amount of waste solution and requires a large amount of energy for drying of produced toner particles. This may potentially increases the production costs. For this reason, an environment-friendly toner production process has been sought after in view of pollution of water resources and generation of carbon dioxide.
As a toner production process using a supercritical fluid, for example, Japanese Patent Application (JP-A) No. 2001-312098 proposes a method for producing toner particles using RESS (Rapid Expansion of Supercritical Solutions) technique. This technique, however, is applicable to only resin that can be dissolved in supercritical fluids, and provides a narrow range of choice of applicable resins. For example, the solubilities of high-molecular weight ingredients or gels (called H body) needed in the toner are of extremely low solubility. In addition, inexpensive and potent styrene-acrylic resins and polyester resins that are generally used in the toner field are also of extremely low solubility. Thus, there is a problem that they cannot be used as they are.
To solve the foregoing problems Japanese Patent Application (JP-A) No. 2004-161824, 2004-144778 and 2005-107405 propose a technique in which rather than dissolving resin in a supercritical fluid, colored resin that has been previously melted and kneaded is granulated by application of shearing force using a dispersing agent. This technology has a problem that it broadens the particle size distribution, though a wide range is ensured for the choice of materials. In particular, broader particle distributions are a critical drawback for obtaining high-resolution images as required by recent toners.
No toner production process has been provided that ensures a sharp particle size distribution and excellent toner characteristics (e.g., charging properties, environmental impact, and temporal stability), creates little waste solution, produces toner containing no monomers left over, and requires no drying process. Likewise, neither a toner produced by this toner production process nor an image forming apparatus using the toner have been provided.